Investigation on the effect of void fraction on annular fuel flow distribution ratio

Annular fuel assemblies feature a dual cooling channel configuration (inner and outer channels), where void fraction variations significantly affect channels flow distribution as reactor power increases. To address this thermal-hydraulic challenge, a 1/8-scale subchannel analysis model was established using practical geometric parameters of the target assembly. The flow distribution model embedded in the SAAF code was validated against out-of-core hydraulic test data of annular fuel assemblies, with relative errors ≤ 3% for key parameters (flow distribution ratio, pressure drop). Systematic investigations across three boiling stages revealed that the outer channel reaches the departure from nucleate boiling (DNB) threshold prior to the inner channel, and void fraction-induced flow redistribution reduces the hot-channel flow distribution ratio by 10%. To enhance thermal safety margin, a higher proportion of coolant flow should be allocated to the outer channel in the thermal-hydraulic design phase. The findings provide actionable technical guidance for the optimal design and safe operation of annular fuel assemblies in advanced PWRs.